(1) Field of Invention
The invention relates to a radio frequency identification system provided to control access to rooms, objects or to workstations and comprises an interrogator, at least one transponder and a protocol according to standard ISO 15693 for passive communication between the interrogator and the transponders, in the sense that control may be performed even if a working distance of an interrogator lying just beyond one meter.
(2) Description of the Related Art
A control over access to rooms, objects or workstations like computers is most usually implemented in a contactless way by means of radio frequency identification (RFID) technology.
The RFID system consists of an interrogator, at least one transponder and a protocol for communication between the interrogator and the transponders. The interrogator is placed at the access point to the controlled object and users authorized to access have suitable transponders.
Communication in most RFID systems is carried out at a frequency of 13,56 MHz according to the protocol subjected to standard ISO 15693. A working distance between the interrogator and the transponder lies below 70 centimeters, usually ranging from 10 centimeters to 40 centimeters only. The transponder receives a strong enough supply flow of the magnetic field energy originating from the interrogator just at such short working distance therefrom. The reason for such short working distance is a low sensitivity of the receiving part of the interrogator. Therefore, the user must bring the transponder closer to the interrogator to the mentioned short distance. However, this is not comfortable and the users need to stop at the interrogator.
Actually, RFID systems with communication according to the protocol subjected to standard ISO 15693 are also known, in which systems the transponders are provided with a battery. Yet just additional functions are made possible like additional sensors and recording the presence of the interrogator field.
A radio frequency identification system of this kind is described in Klaus Finkenzeller, RFID Handbook, John Wiley and Sons Ltd., 2003 (pages 275 and 312) and is schematically represented in FIG. 1. It comprises an interrogator Interrog, at least one transponder Transp and a protocol subjected to standard ISO 15693 for the communication between the interrogator Interrog and the transponders Transp, which communication is passive in the direction from each transponder Transp towards the interrogator Interrog. The transponder Transp is not provided with an energy source and therefore communicates with the interrogator Interrog in a passive way in that a transponder rectifier Rect and a transponder clock extractor ClExtr get power and a clock signal needed for the functioning of the transponder Transp out from electric current being generated by the voltage, which the magnetic field BItransm generated by an interrogator antenna AI induces in a transponder antenna AT. The interrogator antenna AI then detects an impedance variation due to modulation carried out in the transponder Transp and transmitted to its surroundings by the transponder antenna AT.
The interrogator Interrog comprises a data interface DatInterf, which on the one hand provides for a data connection to a system level and on the other hand exchanges data with an interrogator logic circuit PassCommProtImplLCI to implement the passive communication protocol. The interrogator logic circuit PassCommProtImplLCI for implementing the passive communication protocol transmits a signal to an interrogator amplitude modulator AMI, to whose second input an output signal of a high-frequency signal generator HFGen is conducted. An output signal of the interrogator amplitude modulator AMI as a signal to be transmitted is conducted through an antenna matching transformer AMatchTransf to the transmitting and receiving interrogator antenna AI.
In the mentioned way, the transmitting and receiving interrogator antenna AI receives the magnetic field BIrec modulated by the transponder antenna AT and generates a received signal, which is conducted through the antenna matching transformer AMatchTransf and an interrogator amplitude demodulator ADemI to the interrogator logic circuit PassCommProtImplLCI to implement the passive communication protocol.
On the one hand the transmitting and receiving transponder antenna AT is connected through a transponder amplitude demodulator ADemT to a transponder logic circuit PassCommProtImplLCT to implement the passive communication protocol. Said circuit exchanges data with a memory device Mem. On the other hand a signal to be transmitted by the transponder is conducted from the transponder logic circuit Pass-CommProtImplLCT to implement the passive communication protocol through a transponder amplitude modulator AMT to the transmitting and receiving transponder antenna AT.
According to the protocol for the “passive” communication between the interrogator Interrog and the transponders Transp, the interrogator Interrog turns on its high-frequency signal generator HFGen and the transmitting and receiving interrogator antenna AI starts generating the magnetic field BItransm having a constant frequency. After a time interval Δt of at least 1 milliseconds over, the interrogator Interrog transmits an inventory request InvReq that the transponders Transp shall present themselves. Said request can be responded only by the transponders located within the interrogator working distance. Afterward each transponder Transp transmits its own unique identification code within a specified timeslot.
Moreover, a radio frequency identification system is known that operates at ultrahigh-frequency of 900 MHz or 2.4 GHz according to standard ISO 18000-6 A, B and C. A working distance for a passive transponder amounts to a few meters and is even increased if the transponder is provided with a battery. Despite the working distance being very adequate such system is not suitable to carry out access control in the vicinity of electrically partially conductive objects, a human body being as such at these frequencies as well.
A radio frequency identification system operating at a frequency of 433 MHz is also known. The interrogator and the transponder are active radio devices communicating in a digital form and according to the protocol subjected to standard ISO 18000-7, which is similar to the protocols foreseen for the RFID systems. The working distance is similar to these of low-power radio devices, i.e. about fifty meters. Such RFID system is not suitable for access control, since the working distance may not exceed three meters.
No known radio frequency identification system suitable for access control can operate at working distances within an adequate range. The adequate range of working distances for the radio frequency identification system performing access control is defined in the following way: the system must operate at a distance exceeding one meter and, due to security requirements, the working distance should not exceed three meters. Furthermore, the system should allow access even when the transponder battery is run-down, yet only at a short distance. It is desirable, however, that an improved system of this type be compatible with the existing systems of this type.